The present invention related to calenders, and in particular to a magnetic calender provided with a device for adjusting the contact pressure between the rolls.
The calender is know to be a machine manufactured in many different types for exerting a laminating action on various types of materials (paper, rubber, fabrics, etc.), which as finished product are in the form of continuous sheets or tapes. By means of a calender it is possible to flatten, shape, couple, taper or superficially finish in various ways one or more sheets of material passed through a pair of counterrotating parallel rolls. Such rolls are pressed against each other with a variable load depending upon the type of material and of process to be carried out, and they are generally hollow so as to allow their heating and/or cooling by means of an inner circulation of fluid.
In prior art calenders the contact pressure between the rolls is generated by driving them against each other by means of mechanical or hydraulic devices acting at the rotation and support pivots. However, this conventional arrangement does not ensure the uniformity of the pressure along the entire contact line, in particular for rolls of some length. In fact, the thrust exerted on the pivots obviously decreases upon increasing of the distance from the support, so that the rolls are generally convex at the center in order to make up for the decreased thrust in said area.
This conventional solution has various drawbacks affecting both the machine working and the product quality. First, the non-uniform pressure causes vibrations and accordingly noisiness, as well as a consequent non-uniform wear of the rolls. Furthermore, the control of the contact pressure is neither easy nor precise because they may be an excessive pressure at some points and/or an insufficient pressure at some others.
As far as the product is concerned, the material being pressed with a pressure varying along its transversal dimension is not perfectly uniform. Moreover, in a determined point the pressure may also change in time due to the irregular wear, to settling, etc. so that a batch of material may be treated in a slightly different way than the subsequent one.
A partial overcoming of these drawbacks is achieved by the magnetic calender, wherein the contact pressure between the rolls is obtained by means of a magnet generating a magnetic flux which is constant along the whole length of the rolls, which are made of a paramagnetic or ferromagnetic material. The magnet is provided with pole pieces extending along the whole length of the rolls so as to achieve an almost constant pressure along the line of contact between the rolls.
However, even this arrangement does not provide the best operation in the most common case where the calender is used to process materials which may have different widths, whereby the rolls do not work along their whole length but just on the length contacting the material. As a result, in the end portions where there is no material to be pressed the rolls tend to touch thus causing an unevenness of the pressure. In fact, the closing in of the rolls causes a reduction of the gap towards the polar pieces and therefore an increase in the magnetic induction, with a subsequent increase in the force of attraction between the rolls which is proportional to the square of the induction.
In practice, the phenomenon which occurs is the so-called xe2x80x9cmagnetic instabilityxe2x80x9d which causes a change in the contact pressure and therefore unevenness in the processing of the material.
In order to adapt somewhat the calender to the width of the material to be processed, magnets have been devised which are divided into a plurality of independent mobile portions. However, such an arrangement has a significant mechanical complexity and implies a quite difficult adjusting procedure. Moreover, the results which can be achieved are not very satisfactory in that there is no control on the actual evenness of the pressure and the above-mentioned magnetic instability phenomenon can not be prevented.
The object of the present invention is therefore to provide a magnetic calender provided with a device for adjusting the contact pressure which overcomes the aforementioned drawbacks.
Such an object is achieved by means of an adjusting device which detects the flux changes in each of the cores into which the roll-pulling electromagnet is divided, and adjusts the intensity of the magnetomotive force (m.m.f.) of each core so as to prevent the magnetic instability and generate a constant and uniform magnetic flux along the whole roll length.
The main advantage of the device according to the present invention is therefore that of generating a perfectly even and constant contact pressure between the calender rolls thanks to the xe2x80x9ctrimmingxe2x80x9d capacity provided by the adjusting device. In this way the vibrations, the noisiness and the production defects are greatly reduced and the wear as well is uniform. Furthermore, a high treatment reproducibility is obtained, so that consecutive batches of material are far more homogeneous since the production conditions are constant.
A second advantage of the present calender is that it achieves a perfectly even heating of the rolls due to the eddy currents (so-called Foucault""s currents) which are generated on the surface of the rolls through the effect of the magnetic field rotating with respect thereto. Since such a heating is uniform along the contact line, it contributes as well to make the working conditions uniform.
Moreover, this adjusting device is much simpler and cheaper to manufacture than an mechanical device, and it allows to easily implement an automatic system for process adjusting and for recording the working parameters.